1. Field
The present disclosure relates to a method and an apparatus for an embedded isolation filter. In particular the present discourse provides for a method and an apparatus for an embedded isolation filter embodied in a printed circuit board (PCB). As noted, applicant's previously filed patent application Ser. No. 12/655,834 when attaching an electrical component to the bottom side and/or the top side of a printed circuit board (PCB), there is the problem of power loss due to the distance of the capacitance to the points on a corresponding IC for which it is intended. It is not possible to physically locate the capacitance directly to the contact pads on the printed circuit, which corresponds to the input output points of an integrated circuit or, in the case of a test board, the corresponding points of the test socket. Similarly, there is the problem of inadequate power dissipation due to the distance of a resistance to the electrical component. Again, it is not very likely to physically locate the resistance at the contact pads on the printed circuit board. Applicant's previous patent application Ser. No. 12,655,834 addresses this problem by providing a solution for attaching and embedding a capacitance or a resistance directly to an adaptor board or an interposer board that is then connected to the main circuit board. The adaptor board could then be connected to the main circuit board by soldering, electrically connecting it by a conductive elastomer connection, spring pins or by any other way that is known in the art. There is a need however to provide for an embedded isolation filter notably that can be used in analog and RF applications. An embedded isolation filter can also have broader uses such as reducing noise coupling on RF supplies without adding board spaces.
2. The Related Art
RF input designs commonly set RF gain by drawing a small DC current from the DC power supply to the RF transistor circuit. This allows the RF supply to be have a relatively higher impedance than, for example, its digital counterpart. It also aids printed circuit board layout, as it is very difficult to maintain an extremely low impedance so close to the RF device.
However, the DC current biasing the RF input design is very susceptible to interfering noise. This is depicted in FIG. 1. In FIG. 1, the RF device interfaces to the 5 Vias on the left. Via #3 and #4 are the DC supplies for the RF circuit. If Via #2, for example, is noisy due to much higher currents, it has the opportunity to couple into the RF supply pin. Such noisy higher currents are commonly generated in RF devices from, for example, phased locked loops. Components to remove the noise must be located some distance away, as shown on external Vias #1-#3. The components—often two inductors and one capacitor—form a noise reduction filter for DC bias currents known as an isolation filter.
As the connection pin pitch of such RF devices gets smaller, this problem worsens, simply due to greater proximity to the noisy nodes. This higher density also forces boards to be thicker, which results in much longer Vias. Such longer Vias have more coupling potential.
When such noise couples on an RF supply, it creates an unwanted modulation between the desired input signal and whatever noise couples into the device. This unwanted modulation can have several negative effects, such as inter-modulation distortion (e.g. measurements IP2, IP3), baseband signal to noise distortion, baseband integrated noise, and noise figure. The end result is that the RF receiver does not perform as well.